Also there's a twitter thread.
From the introduction:
In the process of reading the book and encountering some extraordinary claims about sleep, I decided to compare the facts it presented with the scientific literature. I found that the book consistently overstates the problem of lack of sleep, sometimes egregiously so. It misrepresents basic sleep research and contradicts its own sources.
In one instance, Walker claims that sleeping less than six or seven hours a night doubles one’s risk of cancer – this is not supported by the scientific evidence. In another instance, Walker seems to have invented a “fact” that the WHO has declared a sleep loss epidemic. In yet another instance, he falsely claims that the National Sleep Foundation recommends 8 hours of sleep per night, and then uses this “fact” to falsely claim that two-thirds of people in developed nations sleep less than the “the recommended eight hours of nightly sleep” – a myth that spread like wildfire after the book’s publication.
Walker’s book has likely wasted thousands of hours of life and worsened the health of people who read it and took its recommendations at face value.
Any book of Why We Sleep’s length is bound to contain some factual errors. Therefore, to avoid potential concerns about cherry-picking the few inaccuracies scattered throughout, in this essay, I’m going to highlight the five most egregious scientific and factual errors Walker makes in Chapter 1 of the book. This chapter contains 10 pages and constitutes less than 4% of the book by the total word count.